Im not entirely sure (im just saying from experience) but I got a new 2 year old horse about 2 months ago (i own 8 others though ;) ) and didnt want to change his diet incase it led to problems like you said. He was previously on a list of other foods, so I slowly changed them into his food system, and kept some of his old foods the same.
But in your case if she has only just had hay, you could slowly introduce chaff, and then whatever type of food you want to feed her! :) I feed my guy lotsss different food such as chaff, sunflower seeds, soya meal, copra, stud gro, livamol, and equilibrium for his minerals and some few herbs, and my guy is doing great.
So I guess keep her on her hay and just slowly introduce new feed into her system.
Hope this helps :)